I find that pee balls are fairly round and diarrhea will form a "squashed", flat lump. But to be sure, you can use toothpicks to break the ball apart to really see what it is made of. I know, not a pleasant task but when I want to be sure, that's what I do!

Yeah this is the problem! He has been having regular poop in there as well. And I did find some odd "squashed" litter lumps but when i used the litter scoop to break them in half, they looked identical to the cross section of a pee "ball" (which I broke in half) to me @_@